This commit addresses comment in the code review. Now, instead of
injecting an slog.Logger we can inject any method that can use a more
flexible implementation.
This commit allows logging errors in a slog.Logger injected in the
context. This type of logger is not currently used directly in step-ca,
but this will change in the future.
When running with `STEPDEBUG=1`, a response with a `StackTracedError`
would result in a nil pointer error. This commit fixes the check and
adds a test case.
* api/log: initial implementation of the package
* api: refactored to support api/log
* scep/api: refactored to support api/log
* api/log: documented the package
* api: moved log-related tests to api/log
